First, the special problems of screw extrusion compounding and fabrication processes and, second, problems of uniformly dispersing heterogeneous materials (particulate fillers, fibrous reinforcement, colorant pigments) throughout a polymer matrix in a plasticated fluid state, generally have preoccupied technical attentions to date. Consequently, relatively little innovation has been dedicated to the special needs of reciprocating-screw injection molding and injection-blow molding processes, especially with homogeneous thermoplastic. One such example use now emerging is injection molded 2D and 3D electrical circuit board substrates or connectors based on newer high performance engineering thermoplastics. PEI (polyether imide), PES (polyether sulphone), PEEK (polyether etherketone), PPS (polyphenylene sulfide) and the Liquid Crystal Polymers commonly require extraordinarily high melt processing temperatures (600-800.degree. F.). Reference is made to "Keys to Predicting Processibility of Engineering Thermoplastics", Plastics Technology, Apr., 1986, 89-92, FIGS. 1 and 2. This creates a delicate balance between thermal degradation/depolymerization on the one hand and insufficiently low melt viscosity on the other hand (unable to freely flow into tight spaces and around numerous pins, without great molded-in stresses, knit lines or orientation-induced shrinkage variations, all of which are revealed in subsequent plating or high-temperature soldering operations), either condition leading to bad product.
Of particular interest herein is means for attaining optimal melt quality in reciprocating-screw injection molding of optically-clear thermoplastic products such as ophthalmic spectacle lenses, visors and goggles, information-storage optical disks for audio, video, or computer, and precision optical lenses for instruments and equipment. The remaining examples of the present invention shall focus upon such reciprocating screw optical molding of transparent thermoplastics, though the invention is not so limited.
The particular needs and requirements of this field can dictate substantially different approaches and theories commonly employed in other screw-plastication apparatus and methods. Most technical papers, publications, and issued patents involving thermoplastic plastication are directed towards extrusion processes, not injection molding. Until very recently, screw designs and other improvements to plastication apparatus had to take into account the needs of two entirely different functions performed by the extruder: (1) melting the thermoplastic; and (2) pressurizing the resulting melt, in order to drive it uniformly through the extrusion die.
Other factors in extrusion screw-and-apparatus design involve, for example, the compounding or intensive mixing function (especially when heterogeneous materials are combined); energy efficiency of the process; maximizing output (pounds per hour), stabilizing output (minimizing variation in die swell, or reducing variations in instantaneous output per unit of time); and preventing thermal degradation or polymer scorch by excessive melt temperatures and excessive residence times (with shear being a major contributor thereto). As a result, most extrusion processes have placed high priorities on such parameters and variables, but these represent inherent trade-offs or compromises against achieving optimal melt quality and freedom from unmelt. Thus, much of the "conventional wisdom" of plastication, being derived as it is from extrusion processes, directly contradicts the needs of those screw injection molding processes which require the best possible melt thermal uniformity and visco-elastic homogeneity consistent with minimal polymer degradation, such as is required by the specialized processes for precision optical injection molding of thermoplastics.